Forgive my ignorance, but I feel that anyone who said Cannabis has not experienced a true bad trip. I'm not talking about a panic attack, I'm speaking of a ride to hell and back. Truly, Cannabis can not create anything like the immense pain and suffering that I experienced on acid. A real bad trip, nothing like, "Oh, weed boosts my anxiety and I get really scared".
But to answer the OP, from all of the information I have encountered over the years and experienced, I would say the average person is more likely to have a bad trip on shrooms. It is in fact, the only drug I have ever had a bad trip on while in a positive state of mind.
That's my two cents.